STOCKHOLDERS' EQUITY (DEFICIT) - Summary of Common Stock Issued and Reserved for Issuance (Details) - shares	Mar. 31, 2020	Dec. 31, 2019
Class of Warrant or Right [Line Items]
Common stock, shares issued (in shares)	109,991,553	106,801,409
Shares of common stock reserved for issuance for:
Total shares of common stock issued and reserved for issuance (in shares)	147,986,000	144,713,000
Warrants
Shares of common stock reserved for issuance for:
Shares of common stock available for issuance (in shares)	22,895,000	22,895,000
Stock options
Shares of common stock reserved for issuance for:
Shares of common stock available for issuance (in shares)	9,704,000	6,236,000
Shares available for grant under 2014 Stock Incentive Plan
Shares of common stock reserved for issuance for:
Shares of common stock available for issuance (in shares)	5,370,000	8,753,000
Shares available for sale under 2014 Employee Stock Purchase Plan
Shares of common stock reserved for issuance for:
Shares of common stock available for issuance (in shares)	25,000	28,000